Thermal decomposition of hydrocarbons



April 19, 1938. p, SEGUY 2,114,544

THERMAL DECOMPOSITION OF HYDROCARBONS Filed Jan. 31, 1930 Jean Z). Syn/vATTORNE Patented Apr. 19, 1938 UNITED STATES PATENT OFFICE THERMALDECOMPOSITION OF HYDRO- CARBONS ware Application January 31, 1930,Serial No. 424,755

6 Claims.

This invention relates to the thermal decomposition of hydrocarbons bycracking, and more specifically provides an improved method for heatingthe hydrocarbons during the process of conversion.

The usual method of heating hydrocarbon oil to conversion temperature ascommonly heretofore practiced in the oil industry comprises introducingthe hydrocarbon oil into a continuous 10 tubular heating coil at thecoolest point in the heating zone and gradually bringing the oil up tothe desired conversion temperature as it passes through the coil,finally ejecting it therefrom at the hottest point, or at least at azone in the heating element higher in temperature than the point ofintroduction.

In the cracking industry the present trend is toward relatively highconversion temperatures and relatively short-time exposure to these tomperatures. With the heating method commonly in use, however, the timefactor required to bring the oil to the desired high conversiontemperature is necessarily long and in subjecting the oil to thisgradual heating, transitory cracking, that is,

cracking within the heating element, is quite likely to occur and isusually accompanied by over-decomposition of the heavier constituents inthe charging stock with a resultant deposition of coke in the tubes andthe formation of an excessive amount of fixed gas.

Moreover, I have found, especially with reference to some of therecently developed cracking processes wherein relatively hightemperatures are employed, that fairly definite conditions oftemperature, pressure and time are required to produce the desired finalproducts from any given charging stock and the results achieved will bein proportion to the accuracy with which these three conditions arecontrolled. This is especially a true in processes developed for theproduction of highly anti-knock motor fuels wherein I have found itdesirable, for best results, to arrive at a definite maximum temperaturein a minimum period of time and to hold the oil at or slightly below 5this maximum temperature for a definite period of time.-

My invention provides a method of subjecting the oil undergoingtreatment to a high rate of heat in-put by passing it, preferably athigh 50 velocity, through an element heated by radiant energy resultingfrom the combustion in the furnace and thus bringing the oil quickly tothe .desired maximum temperature.

The invention further provides a method of holding the heated oil at apre-determined temperature for a definite period of time. This may beaccomplished by passing the oil from the radiant heat zone of thefurnace into a convection heat zone, which may be located in the same orin a separate furnace and deriving its heat wholly or in part byconvection from the products of combustion leaving the radiant heatzone. The pre-determined temperature of the oil in the convection heatzone may be substantially the same or a slightly lower temperature thanthat temperature at which it leaves the outlet of the radiant heat zone.In some instances it may be necessary to supply a certain amount ofadditional heat to the convection heat zone and'provision is thereforemade for direct firing in this zone when and if required. On the otherhand, the heated combustion gases from the radiant heat zone may behigher in temperature or greater in quantity, or both, than is requiredfor holding the oil in the convection heat zone at the desiredtemperature. In this case portions of the combustion gases may by-passthe convection zone and/or those gases which enter the convection zonemay be first cooled by introducing air or steam or any other suitablecooling or thinning medium into the stream.

It is evident that in the manner described, I am able to control thetime and temperature conditions and the time-temperature relation inboth heating zones to a high degree of accuracy and yet, by the use ofthe two zones, allow a greater flexibility of operation than isattainable in the ordinary furnace where a more or less fixedrelationship must prevail between the conditions in the radiant heatsection and in the convection heat section.

The attached drawing is a diagrammatic cross sectional elevation of one.specific type of heating means suitable for carrying out my improvedmethod of heating, but it should not be construed as limiting theinvention to this or to any other specific design or type of furnace orheating element.

Referring in detail to the drawing, l indicates a radiant heat zone, inwhich is disposed a continuous tubular coil 2 of radiant heat tubes. Theconvection heat zone 3 may be connected to the radiant heat zone I by atunnel or gas passageway 4. Fuel for combustion may be supplied by anysuitable type of burner (not shown) to the radiant heat section Ithrough port 5, energy being radiated to the tube bank 2 from the flamesand from the refractory floor 8 as well as from the top and side wallsof this section of the furnace.

Fuel for the heating or the oil through the eonvection coil !3 isobtained by means of a suitable burner (not shown) in the combustiontunnel ll through port I2. By this means the amount of heat imparted tothe oil through the convection coil 13 may be regulated entirelyindependently from the amount of heat imparted to the radiant coil 2.

On the other hand, if it is desired to add heat to or increase thequantity of the combustion gases passing from the convection coil it,all or a portion of the products of combustion from the radiant heatsection I may pass over a bridge wall 1 and through the tunnel 4 to theconvection heat zone 3. Regulated portions of the combustion gases fromthe radiant furnace I may, when so desired, pass from the tunnel 4 up tostack 8 in quantities regulated by the damper 9. All of the gases fromthe radiant furnace I may pass up the stack 8 as above indicated. Ifdesired, a cooling medium, such as air, steam or the like, may beintroduced into the tunnel 4 through the port ID, to mix with thecombustion gases passing through said tunnel on their way from theradiant furnace I to the convection furnace 3, if such operation iscarried out. Thus, the products of combustion from the radiant heat zoneI, all or in part, with or without the addition of a cooling medium andwith or without additional combustion products, may pass upwardly aroundthe tubes in the convection heat bank l3 and finally through the fluel4, past the regulating damper l5 and up the stack 8. Or, as heretoforedescribed, the two coils 2 and I3 may be heated entirely independentlyfrom each other.

The oil traversing the two heating elements may flow in any of a varietyof paths, for example as follows:

The oil to be heated and converted may be introduced into the radiantheat tubes, in substantially liquid state, or in substantially avaporized state, or in a mixed vapor and liquid state, through line l6and the path of the fluid may criss-cross back and forth betweenadjacent tubes of the inner and outer rows of the vertical portion ofthe bank to a point l1. At this point I! the direction of flow may bechanged from a substantially vertical to a substantially horizontalplane and the fluid may flow in a staggered path from each tube in theupper row of the horizontal section to the adjacent tube in the lowerrow, back to the next adjacent tube in the upper row and so on to apoint la. The fluid may then pass through line 19 to an inlet point 20in the top row of the convection tube bank l3, thence flowing from lefthe right through successive adjacent tubes in the top row; transferringto the next lower row, there flowing from right to left throughsuccessive adjacent tubes and so on in a general downwardly directionthroughout the convection bank to the outlet point 2|, from the lasttube. From this point the oil may be discharged through line 22 to othercracking process equipment (not shown).

A number of other paths of flow may be more advantageous with somecharging stocks, and hence the invention is not to be limited to aspecific design or type of heating element and furnace or to a specificcourse or path of flow through the heating element and heating zone,inasmuch as the invention comprises broadly a method of heatinghydrocarbons in a substantially radiant heat zone and in a substantiallyconvection heat zone; the time and temperature on each zone beingindependently controlled.

7 As an example of one of the many operations which may begreatlyimproved by the application of the principles of my invention: A 26 A.P. I. 'Baum gravity topped crude, may be treated in a cracking processwherein reflux condensate from the dephlegmator of the system isrecycled to the heating element for re-cracking. with the chargingstock. The combined feed, at a temperature of about 700 F., owing to theaddition of about four parts of hot reflux condensate to one part ofrawoil, may be heated to an outlet temperature from the heating elementof approximately 915 F., more or less. The heating element may consistof a convection-heat tube bank and a radiant-heat tube bank both locatedin a single furnace. The oil is fed first through a convection bank oftubes where it is gradually heated to a temperature of about 800 F., andthen continues through a radiant bank where it attains a finaltemperature of about 915 F. The heated fluid then discharges into areaction chamber, the vapors from which are subjected to fractionationand their desired light portions condensed, cooled and collected aspressure distillate. An operation of this character may yield approx-,imately 67% of pressure distillate which in turn may containapproximately ofmaterial suitable for motor fuel. This operation beingwhat is known as the non-residuum type the remaining 33% of the chargingstock is coke, gas and loss.

Substituting my method and means of heating in the process abovedescribed: The combined feed may be fed at such a velocity through aradiant-heat tube bank maintained under such temperature conditions thatthe oil is rapidly raised to a temperature of approximately 925 F. atwhich temperature it may be discharged into the convection-heat tubebank in a separate furnace which may be heated by the products ofcombustion from the radiant-heat furnace. The temperature of thecombustion products leaving the radiant-heat zone may be of the order of2,000 E, more or less, and these products, before they pass into theconvection section may be cooled by the introduction of air thereto, toa temperature of about 1600 F. With this controlled temperature in theconvection zone the oil passing through the convection-heat tube bankmay be held at a substantially uniform temperature of approximately 900F. for any predetermined period of time which is controllable not onlyby the design of the heating element with regard to the diameter andlength of tube, but also by the charging capacity or rate of feedthrough the heating element. With the rate of feed maintained the sameas in the first mentioned case, where my invention is not employed, andwith other conditions, except as noted, remaining substantially thesame, the yield of pressure distillate may be increased to approximately75%, based on the raw oil, and this distillate, while containingsubstantially the same percentage of motor fuel as heretofore.mentioned, may yield a motor fuel of higher anti-knock rating. On thisbasis the coke, gas and loss figure may thus'be reduced by the use of myprocess to the extent of 8%, more or less, that is, to about 25%.

It will thus be evident that by employing my method of heating in acertain type of cracking process operating on.a certain charging stockat the usual capacity, the yield and quality of motor fuel may besubstantially augmented and the coke, gas and loss figure greatlyreduced.

I claim as my invention: 7

1. A process for the conversion of hydrocarbon fluids, comprisingflowing hydrocarbon fluid through an elongated passageway of restrictedcross sectional area, heating a portion of said passageway in one zonewith hot gases of combustion, then dividing said gases of combustioninto two streams and discharging one of the streams from the process,subsequently mixing a comparatively cool medium with the remainingstream of gases of combustion leaving said zone, and contacting theresultant gaseous mixture with another substantially posterior portionof said passageway in a second zone.

2. A process for the conversion of hydrocarbon fluids) comprisingflowing hydrocarbon fluid through an elongated passageway of restrictedcross sectional area, heating a portion of said passageway with hotgases of combustion, then diverting a portion of said gases ofcombustion from contact with a further portion of said passageway,mixing a comparatively cool medium with a remaining portion of saidgases of combustion, and contacting the resultant gaseous mixture with afurther portion of said prssageway.

3. A process for the conversion of hydrocarbon fluids, comprisingflowing a hydrocarbon fluid through a tube having a portion arranged ina heating chamber and heating said fluid in said portion to conversiontemperatures, subsequently passing the heated fluid through a conversionportion of said tube arranged in a conversion chamber while maintainingsaid fluid at but not above said conversion temperatures, generating hotgases of combustion and contacting said gases with the first mentionedportion oi? the tube for heating the fluid in the first portion of thetube to said conversion temperatures, passing only a regulated portionof said hot gases of combustion through the conversion chamber, anddiluting the last mentioned gases as they pass into the conversionchamber with cooler diluent gases for maintaining the fluid in theconversion tube at but not above said conversion temperatures.

4. A process for the conversion of hydrocarbon fluids, comprisingpassing a hydrocarbon fluid through a portion of a tube in a heatingzone, generating hot gases of combustion and passing the same in contactwith said portion of the tube to heat the fluid to conversiontemperatures, subsequently passing the heated fluid while at conversiontemperatures into and through a conversion portion of the tube in aconversion zone, discharging a portion of the gases of combustion fromthe heating zone and preventing the same from entering the conversionzone while diverting another portion of said gases of combustion afterdischarge from the heating zone into and through the conversion zone,introducing cooler diluent gases into the gases of combustion enteringthe conversion zone before the gases of combustion contact with theconversion portion of the tube, and causing the mixture of diluent gasesand combustion gases to pass through the conversion zone for maintainingthe'fluid in the conversion portion of the tube at but not aboveconversion temperatures.

5. A process for the conversion of hydrocarbon fluids comprising flowinga hydrocarbon fluid through a tube, heating a portion of said tube inone zone by gases of combustion and thereby cooling said gases ofcombustion, afterwards dividing said cooled gases of combustion into twostreams and passing one stream of said cooled gases of combustion incontact with another substantially posterior portion of said tube in asecond zone, discharging the second stream of gases of combustion andpreventing it from re-entering either of said zones, and tempering thefirst stream of gases of combustion after it has contacted with thefirst mentioned portion of said tube and before it has contacted withthe second mentioned portion of said tube, by introducing relativelycool diluent gases into the first stream oi. gases of combustion as itenters the second zone. I

6. In the heating of hydrocarbon oils. to cracking temperatures infurnaces of the type having a combustion zone and a convection heatingzone, the method which comprises generating hot combustion products inthe combustion zone, passing the oil through the combustion zone andheating the same therein preponderantly by radiant heat to the desiredmaximum cracking temperature, then passing the oil through theconvection zone, removing the combustion products from the combustionzone and dividing the same into two streams, discharging one of thestreams from the process and commingling a relatively cool medium withthe other of said streams, and passing the resultant mixture through theconvection zone in heat exchange relation with the oil flowingtherethrough, the quantity of the combustion products discharged fromthe process and the quantity or cool medium commingled with said otherstream being regulated so that said resultant mixture contains such anamount of heat units as to maintain the oil at cracking temperature "inthe convection zone without increase above said desired maximum crackingtemperature attained by the oil in the combustion zone. JEAN DELA'I'IREBEGUY.

